B11nuclear-magnetic shielding in BN and BP

The ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ nuclear-magnetic shielding (or chemical shift) values of BP and cubic BN were measured at room temperature. With reference to triethyl borate the shifts in ppm are -35 ± 10, and +28 ± 2, respectively, including bulk susceptibility corrections. A new value of +61 ± 10 ppm is also given for the ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ shift of BP with respect to an aqueous solution of ${\mathrm{H}}_3$P${\mathrm{O}}_4$. These shifts, together with previously measured ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ and ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ shifts in the III-V semiconductors, were compared with the predictions of a localized-bond model of the chemical shift using either the bonding description of Nucho, Ramos, and Wolff or the bond-orbital method (BOM) of Harrison and Ciraci. The Nucho et al. description, in which the ionicity of the bond remains constant but the antibonding character varies, does not agree with the data. The BOM, which allows the polarity of the bond to vary but neglects antibonding, gives the main trend of the data. Inclusion of antibonding in the BOM, in an amount depending on Harrison and Ciraci's metallicity parameter, significantly improves the agreement with experiment.